I'd also recommend the add-on for the "crew abilities" that can be purchased or earned. Although a little "gamey," as an old RPG fan, the new abilities add a nice element. For example, you may be able to add an officer to your boat that enhances its dive time, or loads torpedoes faster, etc... A small addition, but a very good one in my mind. And, like others have said, your purchase will send a clear message to developers of sims like these
Cheers,
Howler
p.s. also there are whole new limitations on how you can crew your boat. Limits on how many officers, and the pool from which to choose from is far smaller...adds to the realism...